[builtins] Fix signed integer overflows in divmodsi4.c, divmoddi4.c and divmodti4.c

[Ka-Ka]

Summary

When compiling the builtins with the undefined behavior sanitizer and running testcases you end up with the following warning:

UBSan: divmodsi4.c:22:17: signed integer overflow: 2147483647 - -1 cannot be represented in type 'si_int' (aka 'long')

This can be avoided by doing the subtract in a matching unsigned variant of the type.

The same kind of pattern is found in divmoddi4.c and divmodti4.c

This was found in an out of tree target.

compiler-rt/lib/builtins/divmoddi4.c

 #include "int_lib.h"
 
 // Returns: a / b, *rem = a % b
 
 COMPILER_RT_ABI di_int __divmoddi4(di_int a, di_int b, di_int *rem) {
   const int bits_in_dword_m1 = (int)(sizeof(di_int) * CHAR_BIT) - 1;
   di_int s_a = a >> bits_in_dword_m1;                   // s_a = a < 0 ? -1 : 0
   di_int s_b = b >> bits_in_dword_m1;                   // s_b = b < 0 ? -1 : 0
-  a = (a ^ s_a) - s_a;                                  // negate if s_a == -1
-  b = (b ^ s_b) - s_b;                                  // negate if s_b == -1
+  a = (du_int)(a ^ s_a) - s_a;                          // negate if s_a == -1
+  b = (du_int)(b ^ s_b) - s_b;                          // negate if s_b == -1
   s_b ^= s_a;                                           // sign of quotient
   du_int r;
   di_int q = (__udivmoddi4(a, b, &r) ^ s_b) - s_b;      // negate if s_b == -1
   *rem = (r ^ s_a) - s_a;                               // negate if s_a == -1
   return q;
 }

compiler-rt/lib/builtins/divmodsi4.c

 #include "int_lib.h"
 
 // Returns: a / b, *rem = a % b
 
 COMPILER_RT_ABI si_int __divmodsi4(si_int a, si_int b, si_int *rem) {
   const int bits_in_word_m1 = (int)(sizeof(si_int) * CHAR_BIT) - 1;
   si_int s_a = a >> bits_in_word_m1;                    // s_a = a < 0 ? -1 : 0
   si_int s_b = b >> bits_in_word_m1;                    // s_b = b < 0 ? -1 : 0
-  a = (a ^ s_a) - s_a;                                  // negate if s_a == -1
-  b = (b ^ s_b) - s_b;                                  // negate if s_b == -1
+  a = (su_int)(a ^ s_a) - s_a;                          // negate if s_a == -1
+  b = (su_int)(b ^ s_b) - s_b;                          // negate if s_b == -1
   s_b ^= s_a;                                           // sign of quotient
   su_int r;
   si_int q = (__udivmodsi4(a, b, &r) ^ s_b) - s_b;      // negate if s_b == -1
   *rem = (r ^ s_a) - s_a;                               // negate if s_a == -1
   return q;
 }

compiler-rt/lib/builtins/divmodti4.c

 #ifdef CRT_HAS_128BIT
 
 // Returns: a / b, *rem = a % b
 
 COMPILER_RT_ABI ti_int __divmodti4(ti_int a, ti_int b, ti_int *rem) {
   const int bits_in_tword_m1 = (int)(sizeof(ti_int) * CHAR_BIT) - 1;
   ti_int s_a = a >> bits_in_tword_m1;                   // s_a = a < 0 ? -1 : 0
   ti_int s_b = b >> bits_in_tword_m1;                   // s_b = b < 0 ? -1 : 0
-  a = (a ^ s_a) - s_a;                                  // negate if s_a == -1
-  b = (b ^ s_b) - s_b;                                  // negate if s_b == -1
+  a = (tu_int)(a ^ s_a) - s_a;                          // negate if s_a == -1
+  b = (tu_int)(b ^ s_b) - s_b;                          // negate if s_b == -1
   s_b ^= s_a;                                           // sign of quotient
   tu_int r;
   ti_int q = (__udivmodti4(a, b, &r) ^ s_b) - s_b;      // negate if s_b == -1
   *rem = (r ^ s_a) - s_a;                               // negate if s_a == -1
   return q;
 }
 
 #endif // CRT_HAS_128BIT